The electroless nickel plating industry has long been involved in developing metal coatings for various applications and various substrates. These coatings are deposited on substrates, including metallic and non-metallic substrates, to impart physical and chemical properties of a nickel alloy to the substrate. The electroless plating method typically utilizes reducing agents, such as hypophosphite or boron and is generally characterized as a controlled autocatalytic chemical reduction process for depositing the desired metal alloy on the substrate. The deposit is formed by immersing the substrate into an aqueous nickel plating solution in the presence of the reducing agent and under appropriate electroless nickel processing conditions.
Phosphorus-containing nickel alloys as produced in an electroless nickel plating process are valuable coating deposits having desirable properties such as corrosion resistance and high hardness. Electroless nickel plating employing phosphorus reducing agents such as hypophosphites provide a continuous deposit of a nickel phosphorus alloy coating on metallic or non-metallic substrates without the need for an external plating current.
Electroless nickel is often referred to as “autocatalytic” plating because the metal being applied is in solution and adheres itself to the substrate with the use of an electrical power current. Thus, one of the primary benefits of electroless deposition is that it requires no electricity for metallic deposition. Electroless plating also differs from “immersion” plating in that desired thicknesses of the deposited layer(s) can be achieved in contrast to immersion plating in which coverage with only nominal thickness may be achieved.
Electroless nickel processes are capable of depositing a reliable, repeatable nickel coating of uniform thickness on various substrates, including non-conductive or dielectric substrates such as plastics and ceramics and on metal substrates, including steel, aluminum, brass, copper and zinc. Because electroless nickel is free from flux-density and power supply issues, it is capable of providing an even deposit regardless of workpiece geometry. Thus, it is capable of effectively coating substrates with complex geometries, including sharp edges, deep recesses, internal areas, seams and threads, without resulting in excessive build up on points, corners, etc. In addition, electroless nickel coatings also demonstrate excellent corrosion protection and improved wear resistance as well as good lubricity, high hardness and good ductility.
Electroless nickel may also be used for the coating of non-conductive substrates such as plastic substrates, to render the surface of such substrates conductive and/or to change the appearance of the substrate. Furthermore, by the deposition of nickel, the material properties of the coated substrate can be improved, including corrosion resistance, hardness and wear resistance.
In electroless nickel plating baths employing hypophosphite ions as the reducing agent, the nickel deposit comprises an alloy of nickel and phosphorus with a phosphorus content of from about 2% to more than 12%. These alloys have unique properties in terms of corrosion resistance and (after heat treatment) hardness and wear resistance.
Deposits from nickel phosphorus baths are distinguished by phosphorus content, which in turn determines deposit properties. The percentage of phosphorus in the deposit is influenced by a number of factors, including, but not limited to, bath operating temperature, the operating pH, the age of the bath, concentration of hypophosphite ions, concentration of nickel ions, the phosphite ion and hypophosphite degradation product concentration as well as the total chemical composition of the plating bath including other additives.
Low phosphorus deposits typically comprise about 2-5% by weight phosphorus. Low phosphorus deposits offer improved hardness and wear resistance characteristics, high temperature resistance and increased corrosion resistance in alkaline environments. Medium phosphorus deposits typically comprise about 6-9% by weight phosphorus. Medium phosphorus deposits are bright and exhibit good hardness and wear resistance along with moderate corrosion resistance.
High phosphorus deposits typically comprise about 10-12% (or more) by weight phosphorus. High phosphorus deposits provide very high corrosion resistance and the deposits may be non-magnetic (especially if the phosphorus content is greater than about 11% by weight).
Heat treatment of the electroless nickel deposit (at temperatures of at least about 520° F.) will increase the magnetism of the deposit. Additionally, even deposits that are typically non-magnetic as plated will become magnetic when heat-treated above about 625° F. The hardness of electroless nickel coatings may also be enhanced by heat treatment and is dependent on phosphorus content and heat treatment time and temperature.
It is oftentimes desirable to introduce sulfur into the electroless nickel plating solution to improve properties of the electroless nickel phosphorus deposit. In addition, it is also desirable to produce a nickel phosphorus deposit that is capable of passing the RCA nitric acid test. The RCA nitric acid test is a quality control test that involves immersing an electroless nickel phosphorus coated coupon or part into concentration nitric acid (70% by wt.) for 30 seconds. If the coating turns black or grey during immersion it fails the test.
Deposit porosity and co-deposited contaminants such as sulfur have been found to affect the results. Thus, the inclusion of sulfur in the deposit can cause the coating to fail the RCA nitric acid test.
U.S. Pat. No. 5,718,745 to Itoh et al., the subject matter of which is herein incorporated by reference in its entirety, describes an electroless plating bath for forming black coatings comprising a nickel salt and a reducing agent and further containing a sulfur-containing compound, zinc ions and, optionally, microparticles.
U.S. Pat. No. 3,887,732 to Parker et al., the subject matter of which is herein incorporated by reference in its entirety, describes that the residual stress of a nickel-phosphorus coating deposited on a metal substrate may be controlled.
High level of phosphorus (above about 9% by weight, more preferably above about 10% by weight) and up to about 14-15% by weight are often desirable for certain industrial applications such as memory disks.